Rollup merge of rust-lang#85666 - fee1-dead:document-shared-from-cow,…

… r=dtolnay

Document shared_from_cow functions

library/alloc/src/rc.rs

@@ -1859,6 +1859,18 @@ where
     B: ToOwned + ?Sized,
     Rc<B>: From<&'a B> + From<B::Owned>,
 {
+    /// Create a reference-counted pointer from
+    /// a clone-on-write pointer by copying its content.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use std::rc::Rc;
+    /// # use std::borrow::Cow;
+    /// let cow: Cow<str> = Cow::Borrowed("eggplant");
+    /// let shared: Rc<str> = Rc::from(cow);
+    /// assert_eq!("eggplant", &shared[..]);
+    /// ```
     #[inline]
     fn from(cow: Cow<'a, B>) -> Rc<B> {
         match cow {

library/alloc/src/sync.rs

@@ -2413,6 +2413,18 @@ where
     B: ToOwned + ?Sized,
     Arc<B>: From<&'a B> + From<B::Owned>,
 {
+    /// Create an atomically reference-counted pointer from
+    /// a clone-on-write pointer by copying its content.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// # use std::sync::Arc;
+    /// # use std::borrow::Cow;
+    /// let cow: Cow<str> = Cow::Borrowed("eggplant");
+    /// let shared: Arc<str> = Arc::from(cow);
+    /// assert_eq!("eggplant", &shared[..]);
+    /// ```
     #[inline]
     fn from(cow: Cow<'a, B>) -> Arc<B> {
         match cow {